First off, it should be 1x1x1 meters, don't use 'by'. You should only state the specific dimensions of the chamber if it's important to the containment of the SCP. Is it specifically necessary to keep this SCP in this space? To me, having a space that small seems like it would hinder containment. If you opened up this chamber, the SCP could immediately burn you. As a side note, something as small as 1x1x1 meters seems more like a box than a room.

The room walls, ceiling, and floor should all be 40 in. thick in order to prevent SCP-XXXX from burning through them.

It should be 'rooms' instead of 'room'. This part of the containment procedures also doesn't make much sense to me. You mentioned that, out of all things, the SCP cannot burn concrete. Why would the foundation make the chamber out of anything other than concrete. Do they want SCP-XXXX to burn stuff?

No one is to ignite SCP-XXXX at any moment unless instructed to do so by a level 4 personnel.

I think this could be better worded as "SCP-XXXX is not to be ignited without the approval of at least one Level 4 researcher." I also recommended dialing down that clearance level. This object is keter (although I feel it would fit better as Euclid, but I'll get into that later), but it still isn't dangerous enough to where level 4 authorization is needed. Keep in mind level 4 personnel are people like Site Directors. I think changing it to level 3 would fit better, as that's someone like a senior researcher.

In an event of a breach, please follow Protocol XXXX-WG in order to extinguish SCP-XXXX. In an event that any staff are taken over by SCP-XXXX-A, it is advised to use the modified weapons inside the boxes at each station of the containment room in order to prevent further damage.

Don't mention a Protocol if you're not going to put it in the document, especially if it's in the containment procedures. This document is supposed to inform foundation personnel how to deal with this SCP. How can they do that if the containment procedures refer to a protocol that's nowhere to be found? I see where you're going with the last part, but it's kinda wordy in its current state. I recommend changing it something like "Security personnel guarding SCP-XXXX are to be equipped with water-based weaponry."

SCP-XXXX is a lighter that when ignited, emits a remarkable multicolored flame (currently named SCP-XXXX-A).

Don't use the word 'remarkable' when referring to something in a professional document. All Foundation documents should be as objective as possible. Also, is it really necessary to give the flame of the SCP its own designation? The lighter and the flame are not two separate entities with different properties, the lighter's only anomalous property is the production of the flame.

If SCP-XXXX is ignited for more than one minute, SCP-3091-A will grow exponentially, engulfing anything it touches.

You need to add some more specific information here. You say the flame grows exponentially, but you do not mention the rate. You could say "If SCP-XXXX burns for more than one minute, the flame will double in size every minute thereafter." I think the last part is necessary, as I feel it is common knowledge that fire engulfs anything it surrounds. I suggest adding the sentence "This growth will only stop once the lid of SCP-XXXX is closed or the flames engulf a human." to transition into the next part of the description.

If a subject is engulfed by SCP-3091-A, they would immediately begin to suffer from third-degree burns all over the body, killing the person in less than █ minutes.

Does the time it takes for the SCP to kill you really need a black box? Black boxes are put there because someone doesn't want personnel reading the document to know the information. I can't see any reason as to why the time it takes for someone to die should be blacked out. Another reason is you're opening it up for the reader to say it takes anywhere from 1-9 minutes to die, which contradicts the parts of your article that require the transformation to take place in a short amount of time. I recommend changing to to 'causing death in less than 30 seconds'.

SCP-3091-1 itself does show signs of being sapient, as it will slowly █████ its prey until they die.

Remember, the use of 'as' here means that the second part of the sentence should provide a reason for the first. This is not the case, as "it will slowly █████ its prey until they die." does not provide evidence towards SCP-XXXX-1 being non-sapient. I recommend replacing this sentence with "SCP-XXXX-1 is sentient, but has not shown any signs of sapience."

SCP-XXXX can burn through any material in its path, except concrete, though it is yet unknown why this is the case.

This sentence should be in the first paragraph of the description, which describes the flame, instead of the middle of the second paragraph, where you discuss SCP-XXXX-1.

When touched by water, SCP-3091-1 will try to retreat to SCP-3091. Note that SCP-3091-1 keeps SCP-3091 inside a pocket or other storage area in the host's clothes once it takes control of the subject's corpse, unless that storage area has burned completely, then SCP-3091-1 will hold SCP-XXXX in either its left or right hand.

What do you mean by 'retreat' to SCP-XXXX? Does SCP-XXXX-1 huddle their body around it? Do they hold it firmly in their hand? Do they open the lighter and hold it up to themselves? The 2nd sentence here is quite wordy and confusing. Get rid of the 'Note that' at the start of the sentence, it is improper in this context of the description. I've never heard someones pocket referred to as a 'storage area', so you should probably change that. I think you should change this sentence to something like "SCP-XXXX-1 will store SCP-XXXX in any intact clothing pockets, otherwise grasping SCP-XXXX in a hand."

Even though SCP-XXXX-1 only manifests when SCP-XXXX-A takes control of a host, it seems to have the same conscious with every host. Testing has revealed that SCP-XXXX-1 would try to breach containment the exact same way, kill any staff the same way by [DATA REDACTED], and laugh the same way when it detects its prey.

Addendums are for additional documents or revisions to the document, and this isn't either of those. It would fit better as the last paragraph of the description.

Addendum 2: 2 months and 6 days after initial containment, SCP-XXXX had already figured out a way to escape. D-51767 was called on for testing, and when he entered the containment cell, SCP-XXXX ignited on its own accord for the first time, and SCP-XXXX-A traveled through the air, touching and engulfing D-5176's body. When SCP-XXXX-A took control of D-5176's body, it managed to run past the door before it closed, and attacked everyone outside its containment cell. After the breach alarm was set off, the nearest MTF was summoned to eliminate SCP-XXXX-1. They sprayed SCP-XXXX-1 with specialized water-based projectiles, destroying SCP-XXXX-1. Better containment procedures have been introduced, including a pool of water surrounding SCP-XXXX's containment cell and heat detectors inside the cell that activate water sprinklers inside the containment cell.

I feel a better title for this section of the article would be Incident Log, followed by the date of the incident. Also, why would an MTF team be summoned to take care of this SCP when it is already guarded by people with weapons specifically designed to counter it? Instead of lumping the new containment procedures at the end, you could say something like "New containment procedures enacted. See Revision-XXXX-1." Then, in the special containment procedures, put a sub-section called 'Revision-XXXX-1' and list the new containment procedures there. If you do this, then you can also delete addendum 3.

Now for the more general issues. Besides just having a fancy multi-colored flame, there really isn't anything particularly interesting about this SCP. The closest thing you have to a story is the hint that the SCP has some kind of consciousness that manifests in the people it possess. If you want this to be more than a funky object, you need to flesh out the story more. I personally recommend finding some other way to flesh out the SCP besides the concept I already mentioned, as it is quite similar to SCP-035.

The other thing bothering me is the object class. Keter implies that this thing requires constant, strict containment procedures to not break out. This just doesn't seem to be the case. Even if it can open on its own, all that needs to be done it put it in a concrete container that prevents it from opening. It is still unpredictable though, so it definitely should be Euclid. If you specifically want this SCP to be Keter, then you'll have to change how it works.

The first thing you should do is fix those errors I mentioned, as you want this article to be easy to read and error-free. The next step is to flesh out the story behind this SCP more. Ultimately, a good story is what separates an SCP from an anomalous object.